1. Field of the Invention
The present invention relates generally to a neutron source arrangement in a nuclear reactor for facilitating core startup and, more particularly, is concerned with a mechanism for individually positioning secondary sources in guide thimbles without requiring special modification of the fuel assembly and upper core support plate of the reactor in order to accomodate such arrangement.
2. Description of the Prior Art
A pressurized-water nuclear reactor (PWR) typically includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending between the nozzles and a plurality of transverse grids axially spaced along the guide thimbles. Also, an organized array of elongated fuel rods are transversely spaced apart and supported by the grids between the top and bottom nozzles.
In addition to being structural elements of the fuel assembly, the guide thimbles provide channels for insertion of burnable poison rods, neutron absorber control rods, thimble plugs, and primary and secondary neutron source assembllies into the reactor core. The purpose of a neutron source assembly is to facilitate startup of the reactor core. The primary source spontaneously emits neutrons during initial core loading and reactor startup. After the primary source decays beyond the desired neutron flux level, neutrons are then supplied by the secondary source. It becomes the source of neutrons during periods of low neutron flow, such as during refueling and the subsequent startups.
Historically, in the case of standard PWR design, the reactor core employs four source assemblies: two primary source assemblies and two secondary source assemblies. Each primary source assembly contains one primary source rod and many burnable poison rods. Each secondary source assembly contains a symmetrical grouping of four secondary source rods and several burnable poison rods. The secondary source rods and poison rods are ganged together on, and suspended from, a spider located between the top nozzle and the upper core plate. The spider is held in positiion by a holddown spring which reacts with the upper core support plate.
However, in more recent advanced PWR design which requires secondary sources for reactor core startup, there is no room for a spider between the upper core support plate and the top nozzle. Consequently, a need exists for an alternative technique for positioning the secondary source rods in the guide thimbles other than by the use of a spider which would require significant modification of the fuel assembly and upper core support plate.